SACRAMENTO, Calif. — A coalition of medical marijuana advocates came out Tuesday against a California ballot initiative that would legalize the drug for recreational use and tax its sales.

Proposition 19 would inadvertently harm the most vulnerable patients by allowing local governments to prohibit the sale and purchase of marijuana in their jurisdictions, California Cannabis Association members said.

At a gathering outside the Capitol, the group predicted many cities and counties would impose such bans if voters approve the initiative, leaving local medical marijuana users with few options.

“The people who would be most affected are the sick, the elderly — patients who cannot grow their own and cannot travel to pick up a prescription,” said Amir Daliri, president of Cascade Wellness Center, a medical marijuana dispensary north of Chico.

Supporters of Proposition 19 said it explicitly protects the rights of patients and would provide them with safer and easier access to the drug by creating a strictly controlled, clearly defined legal system for pot cultivation, distribution and sales.

“Proposition 19 is actually going to further clarify that sales of medical cannabis are legal in this state,” said Dale Sky Jones, a spokeswoman for the Yes on 19 campaign. “The intent of our law is to protect medical cannabis patients and their rights.”

If Proposition 19 passes in November, California would become the first state to legalize and regulate recreational pot use. Adults could possess up to one ounce of the drug.

Supporters have targeted two areas of concern for voters: the economy and crime. Legalized pot would bring much-needed revenue to the state and reduce the influence of drug cartels, they said.

The measure was endorsed Tuesday by the largest labor union in the state. The Service Employees International Union, which has 700,000 members, said revenue generated by the initiative would help California preserve jobs and avoid cuts to key services such as education and health care.

The union represents workers in health care, building services and state and local government.

Critics question the economic effects and contend the initiative will simply serve to boost marijuana usage and drug-related crimes.

A Field Poll released in July found 48 percent of likely voters opposed the measure, while 44 percent supported it.